WATER AND NITRATE BUDGETS IN A RENDZINA CROPPED WITH OILSEED RAPE RECEIVING VARYING AMOUNTS OF FERTILIZER

Pollution of the environment by nitrogen (N) has emerged as a serious concern in agriculture, especially in the case of crops such as oilsee d rape. To assess the effect of N fertilization on N dynamics, the mov ements of water and nitrate were determined in a rendzina near Chalons -en-Champagne (eastern France) cropped with oilseed rape with three le vels of fertilizer N and in a bare control. From in. situ micrometeoro logical measurements, actual evapotranspiration rates were computed wi th an energy budget and used to calibrate an evapotranspiration model based on meteorological data and crop leaf area index. Water flow belo w 120 cm was then deduced from periodic measurements of soil moisture contents and precipitation, and the associated nitrate leaching fluxes were calculated from the NO3 concentration measured at the same depth . Denitrification rates and ammonia volatilization were monitored in t he field after fertilizer applications, and crop assimilation of nitro gen was determined frequently during the growth cycle. A nitrate budge t gave an approximation of the in situ net mineralization fluxes. The water balance was influenced by the crop and its fertilization: the cr op's canopy and roots enhanced the water loss by evapotranspiration an d contributed to diminish the soil water storage, whereas drainage vol umes were about the same for all cropped treatments, and significantly greater in the bare soil, The rainy winter was particularly favourabl e to leaching, and losses were much greater (+ 41%) under the over-fer tilized crop than under the non-fertilized one, but remained less (- 4 2%) than those under the bare control soil.